Blowing agents are used extensively in the manufacturing of foam products. Typically, a blowing agent is mixed with a molten base polymer under pressure, and then extruded through a die into a lower pressure atmosphere. The term base polymer, as used herein, refers to any polymer having the appropriate physical properties conducive to the formation of cells containing the blowing agent during the extrusion process used in the production of a polymeric foam. Halogenated blowing agents, such as chloroflurocarbons (CFC) and hydrochloroflurocarbons (HCFC), and in particular, 1-chloro-1,1 difluroethane (HFC-142b), have long been favored as blowing agents in the production of foams containing polystyrene (PS) as the base polymer. It has been suggested, however, that such blowing agents contribute to the depletion of the stratospheric ozone layer. Numerous countries have subsequently signed international agreements to phase out the use of halogenated blowing agents. But finding a replacement for these blowing agents has been problematic.
An acceptable replacement blowing agent should ideally have several properties. They should meet environmental standards by having a low ozone depleting and global warming potential. They should have low or no content of volatile organic compounds associated with the production of smog. They also should have low inhalation toxicity, and low potential as a teratogen, mutagen or embrotoxin. Ideally, the blowing agent is chemically stable and nonflammable. The boiling point and vapor pressure of the blowing agent should be similar to the halogenated blowing agents in current use, so that current foam producing instruments may continue to be used without expensive refitting. For the same reason, the blowing agent's solubility in the polymer should also be similar to the solubility of blowing agents in current use. For applications involving polymeric foams as insulating material, the blowing agent should have low thermal conductivity, have a favorable closed cell size and shape, and low permeability through the walls of the foamed closed cell polymer matrix. Blowing agents having a relatively high vapor pressure will also typically have a relatively high diffusion rate, which can present processing problems and effect properties such as permeability.
Potential candidates for the replacement of halogenated blowing agents in PS foams include hydrofluorocarbons (HFC), hydrocarbons (HC), N2 and CO2. Most HCs are also volatile organics and therefore have the potential to be subject to pollution regulations, necessitating expensive recovery procedures during the production of foams. In addition, the high flammability of HCs also excludes their use in a number of applications, such as insulating foams for building construction. Some HFCs, such as difluoroethane (HFC-152a), are also flammable. Examples of blowing agents are, for example, alkanes; CFCs; HFCs; HCFCs; CO2; N2; air; and mixtures thereof.
CO2, while not flammable, has an unfavorably high vapor pressure, high thermal conductivity, high permeability, and low solubility in PS. These characteristics can lead to processing difficulties such as high pressure drop and high die pressure, and product problems such as poor skin quality, high density and small foam cross-section. A high vapor pressure can also mean a high rate of diffusion of the blowing agent out of the foam and which consequently results in a decrease in insulating properties over time. Certain HFCs, such as 1,1,1,2-tetrafluroethane (HFC-134a), have more favorable properties, with the exceptions of having a low solubility in polymers such as PS and a vapor pressure that is higher than HFC-142b, although not as high as CO2.
Several techniques have been attempted to improve the properties of these candidate blowing agents, including the use of one or more co-blowing agents, the use of additives, or the use of alternative polymers as the base polymer. For example, co-blowing agents may include azeotropic mixtures of HFCs with each other, with CO2 or with HC. But mixtures of HFCs with HC and other organic compounds, such as dimethylether, may still be problematic because of flammability considerations, and such compounds are volatile organic compounds. Mixtures of CO2 and HFCs, or mixtures of HFCs with each other, may still be limited by low solubility and high vapor pressure considerations.
Blowing agents may be combined with additives to modify the permeation of the blowing agent through the polymer matrix, to improve the insulating property of the foam, and to act as a compatibilizing agent. The term “additive” as used herein refers to compounds added to a mixture of polymer and blowing agent, in order to increase the solubility of blowing agent in the polymer. For example, hydrogen bond forming compounds containing ether, ester or ketone groups may associate with hydrogen-containing HCFCs or HFCs and thereby reduce their rates of permeation from the foam. Such compounds may also improve the solubility of blowing agents such as HFC-134a in thermoplastic polymers, such as PS. Additives containing multiple ether bonds or hydroxyl groups, such as polyglycol ether, polyglycol or polyglycerol may increase the solubility of blowing agents, including HFCs, in polypropylene foams. Isobutanol, isopropanol or α-methyl styrene may increase the solubility of HFCs in polyurethane foams. Mixtures of acrylic esters and HFC-134a may have increased solubility in PS.
Notwithstanding these efforts, there remains a need for a method to select additives that can alter the solubility of replacement blowing agents in polymers so as to more closely match the solubility of traditional halogenated blowing agents. The ability to identify additives to facilitate such a matching of solubility's is desirable, so that the replacement blowing agents can be used with a minimum of costly adaptations to existing equipment and processes. Accordingly, what is needed in the art is a method of identifying suitable compounds to improve the solubility of blowing agents in base polymers and a process for their incorporation of such compounds into processes for the production of polymeric foams, while reducing or eliminating the above-mentioned problems.